1. Field of the Invention
The invention relates to an inkjet printing apparatus that uses an inkjet head, and also to a method of controlling the inkjet printing apparatus. Specifically, the invention relates to a technique to effectively remove the mist that is produced when the inkjet head ejects ink.
2. Description of the Related Art
An inkjet head ejects ink directly onto a printing medium from a fine nozzle (hereafter sometimes referred to a unit including an ejection opening, a liquid passage that communicates with the ejection opening and an element to generate energy that is to be used for the ejection of ink). This way of ink ejection sometimes makes the ejected ink bounce off the printing medium, and also generates, in addition to the main mass of ink involved in the actual printing, fine ink droplets (satellites) that are ejected to float in the atmosphere. The bounced back portion of the ink and the floating satellites form ink mists, which sometimes adhere to the portion surrounding the ink-ejection opening of the inkjet head. The ink thus adhered to the surrounding portion of the ink-ejection opening may adversely affects the later action of ejecting ink. Failures of ejection may happen, or color mixture may occur because inks of different colors are mixed together. Consequently, the quality of an image to be printed on the printing medium may deteriorate.
A way of solving the problem is disclosed by Japanese Patent Laid-Open No. H07-195708 (1995). According to the disclosure, the adhered ink is removed by use of a wiping member made of an elastic material such as rubber. The surface of a printing head (hereafter, the surface will be referred to as “ejection face”), in which the ejection openings are formed, is wiped by the wiping member to remove the adhered ink (hereafter, the action of the wiping member will be referred to as “wiping action”). To carry out color printing, an inkjet head may have an array of plural nozzles to eject inks of plural colors. If this is the case, the wiping action may cause inks of different colors to be mixed together on the ejection face. If such color mixture occurs, it is highly possible that the quality of the printed image be unsatisfactory. To address this problem, the apparatus of Japanese Patent Laid-Open No. H07-195708 (1995) performs ejection to discharge inks of predetermined amounts after the wiping action (hereafter, such ejection will be referred to as “a preliminary ejection”), separately from the ejection of inks to actually print an image on a printing medium. While the preliminary ejection leads to an increase in the amount of inks to be wastefully discharged not for printing purpose, Japanese Patent Laid-Open No. H07-195708 (1995) also discloses a technique to suppress such increase due to the preliminary ejection. To this end, the apparatus regulates the number of times of the preliminary ejection on the basis of which kinds of inks have just been used for printing.
In recent years, there have been increasing demands for printing at a faster speed with a higher resolution as well as use of more colors for printing in inkjet printing apparatuses. Such trends lead to increases in the number of ejection openings formed in an inkjet head and in the number of ink colors. Such increases, in turn, tend to increase the amount of inks wastefully discharged in the preliminary ejection. This increase in the wasted inks has called for means of further reducing the wasteful consumption of inks. To this end, a more effective technique has been needed than the technique disclosed in Japanese Patent Laid-Open No. H07-195708 (1995).